Traditional carrier detection methods are generally based on standard energy detection or RMS detection, and require Hi-Q filters, rectifiers, and converters, all of which lead to high power dissipation and a large implementation area, neither of which is desirable.
There is another approach in the prior art, derived from amplitude monitoring techniques involving a simple comparator or a peak detector. FIG. 1 shows a conventional, prior art carrier detector based on amplitude detection. A signal 105 passes through a front filter 102, and the filter output 107 is passed into a comparator 110 having a comparator reference 112. The comparator output 114 is passed to pulse generator 115, then to a latch 117 for a certain period, then finally the signal is output at CD_OUT 120. Pulse generator 115 serves to extend the length of a pulse to enable sufficient time for deglitch module to act.
This architecture is simple, but weak against a transient response of the front filter 102. If the signal 105 is modulated by frequency-shift keying (FSK), the frequency of the carrier changes among multiple frequencies following the data. This generates a discontinuous derivative at the data transition, which causes a fluctuation of the amplitude level at front filter output 107. This fluctuation level depends on the transfer function of the filter 102, which, in the prior art, generally needs a large guard band for amplitude threshold between “detected” and “rejected” amplitudes to compensate for the fluctuation of the amplitude level with various types of filter, reducing the useful amplitude range. For systems with use for applications such as factory automation, the Highway Addressable Remote Transducer (HART) protocol takes direct current (dc) signals and modulates them according to frequency shift keying (FSK) methodologies to permit communication on a do line. The HART Communication Protocol is a hybrid analog/digital industrial automation protocol which has been developed that is extremely useful in overlaying intelligence on otherwise dumb devices. A most notable advantage of the HART protocol lies in permitting communication over legacy 4-20 mA analog instrumentation current loops and sharing pairs of wires used by analog-only host systems.
According to HART, a digital signal may be made up of two frequencies, 1200 Hz and 2200 Hz representing bits 1 and 0, respectively. Digital words can be transmitted by modulating, for instance, digital ones according to a sinusoid at a first frequency, 1200 Hz (“Mark”) and digital zeros according to a sinusoid at a second frequency, 2200 Hz (“Space”). Data transitions, particularly representative of a logic low (“0”), e.g., 2200 Hz (“Space”) transitioning to a logic high (“1”), e.g., 1200 Hz (“Mark”), result in a front filter output of decreased amplitude due to the transfer function of the filter as filter gain is a function of frequency. This decreased amplitude may result in comparator 110 not properly detecting a carrier. A similar, yet, less severe problem may occur for transitions from logic high (1200 Hz, “Mark”) to logic low (2200 Hz—“Space”). In this instance, transient effects may cause the front filter output that is actually higher than an expected threshold level.
FIG. 1A illustrates a timing diagram for the carrier detector of FIG. 1. FIG. 1 illustrates a representative data sequence of logic lows (“0 s”) and logic highs (“1 s”); the frequency shift modulated representation of logic low (2200 Hz—“Space”) and logic high (1200 Hz—“Mark”); a representative filter output of filter 102; and the carrier detector output. Comparator 110 may have trouble detecting data transitions from logic low to logic high, due to low amplitude level output reflected by the front filter transient response during such transitions in connection with a modulation change to the input of filter 102 of, for instance, 2200 Hz transitioning to 1200 Hz. This difficulty is reflected by the circled pulses showing missed carrier detections.
In setting up a system that will properly detect signals in connection with, for example, a HART protocol system, a series of ones, low frequency transmissions are sent to adjust the carrier detect threshold level. Nevertheless, the system may be improperly responsive to spurious signals and noise based on the amplitude level for detection needed for those low frequency signals during carrier detector set-up.
Based on the foregoing, there is a need in the art for a small digital circuit and feedback architecture to establish a small and seamless carrier detector, to reduce fluctuation of the amplitude level at the front filter output.